1. Field of the Invention
The present invention relates to an energy cable. In particular, the invention relates to a cable for transporting or distributing medium or high voltage electric energy, wherein an extruded coating layer based on a thermoplastic polymer material in admixture with a dielectric fluid is present, enabling, in particular, the use of high operating temperatures together and providing the cable with the required thermomechanical properties.
Said cable may be used for both direct current (DC) or alternating current (AC) transmission or distribution.
2. Description of the Related Art
The use of materials compatible with the environment during production or utilization, easily recyclable at the end of their life, is conditioned by the need to limit costs while, for the more common uses, providing a performance equal to or better than that of conventional materials.
In the case of cables for transporting medium and high voltage energy, the various coatings surrounding the conductor commonly are based on polyolefin-based crosslinked polymer, in particular crosslinked polyethylene (XLPE), or elastomeric ethylene/propylene (EPR) or ethylene/propylene/diene (EPDM) copolymers, crosslinked too. The crosslinking, carried out after the step of extrusion of the polymeric material onto the conductor, gives the material satisfactory mechanical and electrical properties even under high temperatures both during continuous use and with current overload.
However, crosslinked materials cannot be generally recycled, so that manufacturing wastes and coating material of cables at the end of their life may be disposed of only by incineration.
Thermoplastic polyethylene (either LDPE or HDPE) was considered for use in medium and high voltage cables, but such polymeric material shows too low an operating temperature (generally about 70° C.).
Polypropylene based thermoplastic materials were considered. In particular, in order to achieve the desired performances, especially in term od dielectric strength and processability, polypropylene materials were considered in admixture with a dielectric fluid.
As reported, for example, by WO02/03398, the addition of a dielectric liquid to an insulating material should both determine a significant increase in its electrical properties (in particular the dielectric strength), without changing the material characteristics (thermomechanical properties, flexibility) and without resulting in exudation of the dielectric liquid. In particular, the resultant cable should provide substantially constant performance with time and hence high reliability, even at high operating temperatures (at least 90° C. and beyond).
WO02/03398 in the Applicants name relates to a cable comprising at least one electrical conductor and at least one extruded coating layer based on a thermoplastic polymer material in admixture with a dielectric liquid, wherein the dielectric liquid comprises at least one alkylaryl hydrocarbon having at least two non-condensed aromatic rings and a ratio of number of aryl carbon atoms to total number of carbon atoms greater than or equal to 0.6, preferably greater than or equal to 0.7. The exemplified compounds have molecular weight higher than 200 g/mole.
WO02/27731 in the Applicants name relates to a cable comprising at least one electrical conductor and at least one extruded coating layer based on a thermoplastic polymer material in admixture with a dielectric liquid, wherein the dielectric liquid comprises at least one diphenyl ether, non-substituted or substituted with at least one linear or branched, aliphatic, aromatic or mixed aliphatic and aromatic C1-C30, preferably C1-C24, hydrocarbon radical.
Said dielectric liquid has a ratio of number of aryl carbon atoms to number of total carbon atoms greater than or equal to 0.4, preferably greater than or equal to 0.7.
WO04/066318 in the Applicants name refers to a cable comprising at least one electrical conductor and at least one extruded coating layer based on a thermoplastic polymer material in admixture with a dielectric liquid, wherein said dielectric liquid has the following characteristics:                an amount of polar compounds lower than or equal to 2.5% by weight with respect to the total weight of the dielectric liquid;        a melting point or a pour point lower than 80° C.;        a ratio of number of aromatic carbon atoms with respect to the total number of carbon atoms lower than 0.6, when the dielectric liquid is aromatic.        